1. Field of the Invention
The present invention relates to a stepping motor drive control apparatus, particularly to a stepping motor drive control apparatus wherein a stepping motor having a plurality of coils is driven in such a way that the coils are switched a plurality of times during one cycle of a step signal.
2. Description of the Prior Art
A stepping motor is widely used as a drive source for various high-precision servo mechanisms. For example, it is used as a drive source for a head seek mechanism which moves record/playback heads in a radial direction crossing tracks of a floppy disc in a floppy disc drive unit for magnetically recording or reproducing information into or from the floppy disc as a magnetic recording medium.
In this head seek mechanism, the above-mentioned type of system is used as a stepping motor drive control apparatus, and a so-called 2-phase, 1-track feed system, wherein the phase of the exciting signal is switched twice during one cycle of a step signal to drive and rotate the stepping motor by a specified angular displacement to move the record/playback heads by one track has been widely used. The conventional arrangement is described in detail below.
FIG. 1 shows an arrangement of a stepping motor drive control circuit in a convenient head seek mechanism.
In FIG. 1, reference numeral 3 designates a stepping motor. By driving the stepping motor 3, a record/playback head 5 is moved in a radial direction of a floppy disc 4, while crossing record tracks on the floppy disc 4.
The stepping motor 3 is driven by a driver 2 under the control of a stepping motor controller 1 having a microcomputer or logic circuits including power transistors.
A power source voltage is applied to the controller 1 and the driver 2 from a power circuit (not shown) through a power line 11. A drive select signal 7 for selecting a disc drive unit assigned to the controller 1 shown in FIG. 1, a step signal 8 which is a pulse signal for driving the stepping motor 3 by one track at a time, and a direction-in signal 9 designating a moving direction (rotating direction of the stepping motor 3) of a record/playback head 5 either as in-direction or as out-direction (inward direction or outward direction of the disc 4) are inputted into the controller 1 from a host system (not shown) via signal lines 7-9.
Assuming here that the stepping motor 3 is of a 4-phase arrangement, that is, it is provided with four coils, and is driven by a 2-phase exciting system in response to these control signals, the controller 1 outputs phase exciting signals .phi.A-.phi.D respectively to the driver 2 via four output signal lines 10 to drive the stepping motor 3.
FIG. 2 is a timing chart illustrating various signals in various portions in FIG. 1. As shown in FIG. 2, a level change or phase shift of the exciting signals .phi.A-.phi.D takes place at instants T0 and T2 synchronized with the leading edges of the step signal 8 as well as at an intermediate instant T1 between the two instants T0 and T2. The exciting sequence is switched according to the level of the direction-in signal 9.
The magnitude of damping (attenuating vibration) during driving of the stepping motor 3 varies depending upon the timing of the intermediate exciting phase switching instant T1. The timing of the switching instant T1 at which this damping is minimized varies in accordance with the period of the step signal 8 or the so-called step rate.
In an arrangement, however, where the timing of the switching instant T1 is set at a specific instant, the instant that the damping is minimized also changes as the step rate changes, but the switching instant T1 cannot be set variably.
For this reason, the conventional system has involved a problem that the motor damping and the noise caused thereby cannot be successfully controlled in response to a variation of the step rate. This will be explained further in detail.
If it happens that the switching point T1 located right in the middle between the switch points T0 and T2 as is the case in FIG. 2, the following relationship is given.
T1-T0=T2T1.
Thus, the exciting times of the respective phases of the exciting signal in one cycle of the step signal 8 or in a one-track movement become equal to each other.
If, however, the cycle corresponding to the pulse rate (frequency) of the step signal 8 increases then the following relationship is obtained.
T1-T0&lt;T2-T1.
Accordingly, a difference occurs between the exciting times of the respective excitation phases in one cycle of the step signal 7, resulting in imbalance of the excitation among the phases. This unbalance increases the damping of the stepping motor 3 producing increased noise.